Certain irregularities have been observed during fertilization after artificial insemination with spermatozoa from male mice carrying tw32, an allele of the T locus. These irregularities may or may not be related to another peculiarity of these males, the abnormal transmission ratio of that allele. It is proposed to make a detailed study of the process of fertilization in vitro using spermatozoa from mice heterozygous of tw32, and to determine the transmission ratio of tw32 in the fertilized ova. This study will examine the timing and morphology of the approach of the gametes, sperm penetration, formation and disappearance of pronuclei, and the first cleavage of the zygote. A cytological marker will be used to detect the presence or absence of tw32 in each zygote. Consistent differences from the congenic controls in the timing or morphology of some part of the fertilization process would indicate how the T locus affects fertilization. Determining the transmission ratio of tw32 at the time of fertilization may help to elucidate the time of distortion of the ratio, and to decide whether or not the ratio distortion is related to the function of the spermatozoa in fertilization. These studies will establish the usefulness of the T locus as a system for studying mammalian fertilization, and may provide insight into the processes involved in fertilization.